Waxy hydrocarbon oils are dewaxed so as to improve their usefulness in any number of applications. Dewaxed oils exhibit reduced cloud and pour points making them useful as liquid lubricants which can operate at low temperature without solidification.
The waxy hydrocarbon oils can be dewaxed in any number of ways.
The waxy oil can simply be chilled to crystallize out the wax and filtered to recover dewaxed oil and a wax product. Low to moderate pour point reductions can be achieved by this technique.
In order to achieve significant pour-point reductions, dewaxing using various solvents has been practiced. Thus waxy hydrocarbon oils can be dewaxed using ketone solvents, such as methyl ethyl ketone, methyl isobutyl ketone, and mixtures thereof; and mixtures of ketones with aromatic hydrocarbons, such as MEK/Toluene. Autorefrigerative solvents such as propane, propylene, butane, butylene, and mixtures thereof can also be used.
Autorefrigerative solvents have been used in batch operation, usually involving two pressure vessels running in alternate sequence.
Autorefrigerative solvent dewaxing involves feeding into a pressure vessel a solution of waxy hydrocarbon oil and solvent under pressure at elevated temperature. The pressure is then slowly reduced permitting the vaporization of the autorefrigeration solvent and resulting in cooling the solution. This cooling causes the wax present in the hydrocarbon oil to crystallize. At completion of the process the slurry of hydrocarbon oil and wax crystals is removed from the vessel and sent to liquid-solid separation means such as filters or a centrifuge wherein the solid wax and oil are separated and recovered as independent streams. The vaporized propane is recovered, repressurized, and mixed with a fresh portion of waxy hydrocarbon oil to produce a charge solution; the solution is then charged to a dewaxing pressure vessel that has been permitted to warm up since the last round of dewaxing.
In order to operate most efficiently, multiple vessels are used and are operated under alternating, batch conditions. Thus, while one vessel is being charged, depressurized - cooled, and emptied, a second vessel that had just previously completed its refrigeration sequence is permitted to warm up before being recharged. In this way a relatively continuous stream of dewaxed oil can be produced in a process otherwise characterized by blocked sequence operation.
This blocked sequence, batch operation has a number of disadvantages.
Operation of the down stream liquid-solid separators, be they filters or centrifuges, cannot be maximized or made most efficient when feed for those separators is fed to them on an intermittent rather than on a smooth, continuous basis.
Batch chilling pressure vessels are fitted with numerous valves that must be maintained and carefully controlled in order to smoothly reduce the pressure on the solvent to achieve controlled chilling and uniform wax particle size, without bumping or boiling of the solvent which can be extremely detrimental to the eventual liquid-solid separation step and dewaxed oil yield.
Because batch chillers must be warmed before oil-solvent introduction and chilling, large amounts of energy are wasted in this temperature cycling.
It would be an advantage if autorefrigerative solvent waxy oil dewaxing could be practiced on a continuous basis avoiding (1) intermittent vessel warm-up, (2) large numbers of control valves, and (3) intermittent flow of oil/crystallized wax to the liquid-solid separation means.